(i) Technical Field
The present invention relates to a method of controlling a wavelength-tunable laser.
(ii) Related Art
Generally, a wavelength-tunable laser oscillates at a desirable wavelength by varying wavelength characteristics of a resonator. Typically, a mirror included in the resonator has wavelength characteristics in order to vary the wavelength characteristics of the resonator. The wavelength characteristics of the mirror are controlled so that a resonant condition of the resonator fits a desirable oscillation wavelength. Another wavelength-tunable laser has a plurality of ring resonators and controls an oscillation condition by controlling the resonant condition. Japanese Patent Application Publication No. 2007-48988 discloses the wavelength-tunable lasers.
A given setting value is provided to each part of the wavelength-tunable laser in order to achieve a desirable oscillation wavelength. For example, a setting value is provided to a gain electrode for setting a gain of the resonator, a heater for controlling wavelength characteristics of DBR (Distributed Reflector) region, and a temperature control device for controlling wavelength characteristics of a DFB (Distributed Feedback) region and a laser oscillation.
A number of wavelength channels is set in a narrow band in a wavelength-tunable laser used in a WDM communication technology. Therefore, an interval of each channel is very narrow. For example, a C-band has 89 channels in a wavelength band from 1528.773 nm to 1563.863 nm. Therefore, a frequency difference between each channel is 50 GHz (approximately 0.4 nm as wavelength difference).
The wavelength channel is defined with a narrow wavelength interval. However, the wavelength-tunable laser has a number of elements to be controlled. Accordingly, only a narrow control range is allowed in a setting value of each element.